High swelling ramming paste for aluminum electrolysis cell

ABSTRACT

A new ramming paste for aluminum reduction cell cathodes is a high swelling cold ramming paste made of a blend of pitch, light oil diluent and an aggregate comprising a mixture of anthracite and crushed anode butts or calcined coke. The presence of the crushed anode butts or calcined coke increases the sodium swelling index of the paste by about four times higher than that of regular ramming pastes. This new high swelling cold ramming paste may also contain an amount of a refractory hard material, such as TiB 2 .

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 10/387,020filed Mar. 12, 2003, now U.S. Pat. No. 7,186,357.

BACKGROUND OF THE INVENTION

This invention relates to an improved ramming paste for aluminumreduction cell cathodes.

Aluminum metal is conveniently produced in an electrolytic cell bypassing a current through a bath of molten cryolite containing dissolvedalumina. The cell is typically a large tank or cell lined with carbon,with the lining serving as part of the cathode system. Large carbonblocks extend into the top of the bath to function as the anode. Moltenaluminum metal collects as a metal pad at the bottom of the cell. Thecell typically has a bottom lining made up of cemented carbon blocks inwhich horizontally disposed steel cathode bars extend from the oppositesides of the cell. These cathode bars are connected to the cathodecollector blocks by means of cast iron, and the blocks are anchored in aside lining.

Most aluminum reduction cells in commercial use employ prefabricatedcarbon blocks as the cell lining and as the cathodic working surface.These blocks provide high operating strength, high density, and lowerelectrical resistivity than that of continuous rammed paste type oflinings. The blocks are formed into a liquid-tight container surfaced byfilling the joints between the blocks with a ramming paste. Theefficiency of sealing of the ramming paste is an important factor indetermining the life and energy efficiency of a reduction cell, whichdepends to a great degree on the extent and rate of electrolyticpenetration into the cell bottom.

The anodes are typically made from crushed petroleum coke and liquidpitch, which is formed into rectangular blocks and baked. These anodeblocks are attached to rods and suspended into the electrolytic cellwhere they are slowly consumed in the aluminum smelting process. Theymust be replaced after several weeks and the remaining portions attachedto the rods that are removed are known as “anode butts”. These butts aretypically recycled.

The cathode blocks are traditionally made from an anthracite aggregatemixed with a pitch binder. Graphite components can be substituted toincrease electrical conductivity. As mentioned above, ramming paste isused to fill the spaces and form seams between individual cathodeblocks, as well as to connect the side walls with the cathode blocks. Atypical hot ramming paste consists of an anthracite filler and a coaltar binder. A cold (room temperature) paste binder usually includes adiluent to lower its softening point.

For making the joints with ramming paste, the paste is added to thejoint spaces and compacted, e.g. with pneumatic hammers. The joints areformed in several layers to a final top surface flush with the top ofthe cathode blocks. Side walls, including monolithic slopes, can beformed in a similar manner by compacting the paste.

Premature cell failure can occur because of infiltration of metal intothe joints, and the problem can be worsened when graphitized cathodeblocks are used, which have a very low sodium swelling index. Because ofthis problem, there is a need for a high swelling, cold ramming pastewhich can serve to tighten the peripheral joint (big joint) and jointsbetween the cathode blocks.

Another problem that can occur with these electrolytic cells is, becauseof occasional excessive metal motion due to magneto-hydrodynamic effects(MHE), the cathode lining and, particularly the amount of monolithicside slope and the joints between the cathode blocks, can be subjectedto rapid erosion and failure.

Reamey et al. U.S. Pat. No. 3,871,986 describes a ramming cement for analuminum reduction cell which is described as not shrinking whensubsequently baked. That required a special pitch binder comprising apetroleum pitch having a cube-in-water softening point between about 40°and about 85° C. and having a content of material insoluble in quinolinenot greater than about 1%.

U.S. Pat. No. 5,961,811 (Keller) describes another form of ramming pastefor an aluminum reduction cell made of carbon and a reactive compound,such as a carbide, fluoride, phosphate or oxide compound, capable ofreacting with titanium or zirconium to produce titanium or zirconiumdiboride during operation of the cell to produce aluminum. The titaniumor zirconium diboride is produced in an amount sufficient to improvemolten aluminum wetting properties of the carbonaceous material.

In Mirtchi, U.S. Pat. No. 6,258,224, the problem had to do with theerosion/corrosion of bottom blocks of an electrolytic cell for producingaluminum because of the movement of cell contents caused by MHE. Thatpatent provided a multi-layer cathode structure including a carbonaceouscathode substrate and at least one layer of a metal boride, e.g. TiB₂,containing composite refractory material over the substrate. The innerface between the substrate and the TiB₂ composite material was firstroughened (raked) to overcome thermal expansion differences between thetwo materials.

Another attempt at improving ramming paste for aluminum reduction cellsis described in de Nora et al. U.S. Pat. No. 5,676,807. The main concernin de Nora was the polluting affect of pitch binders and, according tothat invention, the ramming paste was produced using a colloidal bindingmaterial, e.g. alumina in colloidal form.

It is an object of the present invention to provide an improved rammingpaste that has a high swelling index and can be used under cold, e.g.30° C., conditions.

It is a further object of this invention to provide an improved rammingpaste having good erosion resistance and which is wettable by aluminum.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a novel highswelling cold ramming paste for aluminum electrolysis cells which isreadily prepared from commercially available materials. The paste can beused for the monolithic side slopes and in the joints between thecathode blocks.

It has been discovered that a cold ramming paste typically containinganthracite as aggregate and pitch as a binder, can be provided with ahigh swelling property by adding thereto a portion of crushed anodebutts or calcined coke. The anode butts are the remains of consumedanodes removed from the electrolytic cell and the calcined coke istypically calcined petroleum coke of the type used in the manufacture ofthe anodes.

Thus, according to one main feature of this invention there is provideda high swelling cold ramming paste which comprises a blend of pitch,light oil diluent and an aggregate comprising a mixture of anthraciteand crushed anode butts or calcined coke. The crushed anode butts orcalcined coke typically comprise up to 20% by weight of the totalaggregate, preferably about 15 to 20% by weight of the total aggregate.The pitch is typically present in an amount of about 10 to 15% by weightand a light oil in an amount of up to 10% by weight, e.g. about 5 to 10%by weight, with the balance being the aggregate.

The light oil acts as a solvent for the pitch, lowering its softeningpoint and therefore makes the ramming paste soft and sticky at roomtemperature. The light oil is typically a light carbolic oil residueboiling in the range of about 210-355° C.

According to a further feature of this invention, the above rammingpaste may be provided with erosion resistance as well as wettability byadding thereto a portion of a refractory hard material, e.g. a crushedrefractory material, TiB₂, TiO₂, B₂O₃, SiC, Al₂O₃, etc. These materialscan provide both wettability and reinforcement to the paste. For theramming paste containing a boride such as titanium diboride, a typicalcomposition comprises 30 to 60% by weight titanium diboride, 10 to 15%by weight pitch, 5 to 10% by weight light oil and the balance aggregate,this aggregate being a mixture of anthracite and crushed anode butts orcalcined coke with the crushed butts or calcined coke being present inan amount of up to 20% by weight of the total aggregate. This rammingpaste is referred to hereinafter as “TiB₂-containing paste”.

The components are preferably mixed together to form the paste at amixing temperature of about 90 to 130° C. for a mixing time of about 20to 50 minutes. The paste is preferably applied to the joints between thecathode blocks and to the monolithic slope between the blocks and theside wall of the cell at a temperature in the range of about 20 to 40°C.

The ramming paste is preferably applied to the joints as a series oflayers, with tamping being carried out between layers to form a tight,non-porous joint. The top layer is preferably formed of theTiB₂-containing paste. When the top layer is applied, the layer oframming paste immediately below is not separately tamped and the twolayers are tamped together. This causes intermingling between the layersto allow for thermal expansion differences between the layers. TheTiB₂-containing top layer typically has a thickness of about 2.5 to 12.5cm, preferably about 3.75 to 7.5 cm.

The TiB₂-containing ramming paste typically has a higher density thanstandard ramming paste in both loose and compacted form. In loose formthe TiB₂-containing paste typically has a density at least 25% greaterthan that of regular commercial ramming paste, while in compacted formit has a density typically at least 30% greater.

It is believed that the high swelling property of the ramming paste ofthis invention can be found in the disorganized structure of coke in thecrushed anode butts or calcined coke which constitutes a more favourablesite for sodium penetration and thus sodium swelling. Because of thesodium swelling, a high swelling paste is obtained which can tightenedthe peripheral joint (big joint) and the joints between the blocks, and,therefore, prevent the infiltration of metal and bath. It has been foundthat when crushed anode butts or calcined coke form part of theaggregate in an amount in the range of 15 to 20% by weight of the totalaggregate, the sodium swelling index of the paste is about four timeshigher than that of regular ramming pastes.

Description of the Preferred Embodiments Brief Description of theDrawing

FIG. 1 is plots of expansion as a function of temperature for threedifferent ramming pastes.

EXAMPLE 1

A swelling cold ramming paste was prepared containing 10% by weight ofpitch, 6.3% by weight of a light carbolic oil residue and 83.7% byweight of aggregate. The aggregate contained 83% by weight anthraciteand 17% by weight of crushed anode butts.

For a comparative study, a traditional cold ramming paste was preparedcontaining 10% by weight pitch, 6.3% by weight light carbolic oilresidue boiling in the range of about 210-355° C. and the balanceanthracite aggregate. The properties of these two ramming pastes werecompared to each other and also with the commercial ramming pastes,namely Carbone Savoie AMT73S30 and the Vesuvius CP45.

The properties are shown in Table 1 below. It can be seen that thesodium swelling index of the swelling ramming paste of the invention isabout four times higher than that of the conventional anthracite-basedpaste.

TABLE 1 Ramming Paste Properties New Swelling Std. Cold Cold RammingRamming Carbone Savoie Paste Paste AMT73S30 Vesuvius CP45 Baked Density(g/cm³) 1.456 ± 0.005 1.482 ± 0.007 1.466 ± 0.006 1.542 ± 0.004 (n = 8)(n = 12) (n = 4) (n = 8) Electrical Resistivity 60 ± 1  60 ± 1  66 ± 1 58 ± 1  (μΩ · m) (n = 8) (n = 12) (n = 4) (n = 4) Crushing Strength(MPa) 18.3 ± 0.2  16.5 ± 2.8  18 ± 1  27 ± 1  (n = 8) (n = 12) (n = 4)(n = 4) Tensile Strength (MPa) 1.9 ± 0.2 1.9 ± 0.2 2.2 ± 0.4 3.8 ± 0.3(n = 8) (n = 12) (n = 4) (n = 4) Thermal Expansion 4.48 ± 0.05 4.28 ±0.08 na 4.14 ± 0.12 Coefficient (n = 8) (n = 12) (n = 4) (×10⁻⁶/° C.)Thermal Conductivity at 5.51 ± 0.33 5.58 ± 0.47 na 6.39 ± 0.14 RoomTemperature (n = 8) (n = 12) (n = 4) (W/m ° C.) Shrinkage** 0.23 ± 0.080.27 ± 0.07 0.10 ± 0.06 0.58 (%) (n = 3) (n = 3)  (n = 2) SodiumSwelling Index 4.9 ± 1   1.1 ± 0.1 0.84 ± 0.09 1.2  (%) (n = 4) (n = 6) (n = 2) Tamping Density 1.611 ± 0.014 1.657 ± 0.016 1.622 ± 0.003 1.644± 0.006 (g/cm³)  (n = 15) (n = 16) (n = 4) (n = 4) The results expressedas the average ± the standard deviation n: number of test na: notavailable

EXAMPLE 2

Another very important property of ramming paste is its degree ofshrinkage. The ramming paste of this invention has a shrinkage of lessthan that of Vesuvious CP-45 paste. Typical expansion/shrinkage curvesof the swelling cold paste of this invention, regular Alcan cold pasteand Vesuvius CP45 paste are shown in FIG. 1.

EXAMPLE 3

A further ramming paste was prepared containing titanium diboride. Thispaste was prepared by mixing together 7% by weight light oil, 12% byweight pitch, 55% by weight titanium diboride powder and 26% by weightof aggregate consisting of a mixture of anthracite and crushed anodebutts, with the crushed anode butts being present in an amount of about17% by weight of the total aggregate. These ingredients were mixedtogether at a mixing temperature of about 100 to 120° C. for a mixingtime of about 45 minutes. A ramming paste was obtained which could beused in an operational temperature of about 25 to 35° C.

This greatly increased the erosion resistance of the ramming paste aswell as providing it with wettability with respect to aluminum.

EXAMPLE 4

Tests were conducted to compare the density of the TiB₂ paste of Example3 with a conventional ramming paste (Vesuvious CP-45). Density testswere conducted on both loose material and compacted material.

For the tests, a tube was used having a diameter of 5.22/cm, a height of21.92 cm and a volume of 469.29 cc. For the loose material, the tube wasfilled to the brim, with a couple of raps on a counter to cause somesettling of the paste. In preparing the compacted sample, the materialon the tube was impacted 200 with a tamping piston to cause compaction,after which the density was determined.

The results are shown in Table 2 below:

DENSITY g/cc Vesuvious TiB₂ containing Free Compacted Free Compacted0.847 1.655 1.092 2.194

1. A high swelling cold ramming paste for use in jointing components ofelectrolytic cells for the electrolysis of alumina for the production ofaluminum, comprising a mixture of pitch, a light oil, 30 to 60% byweight of titanium diboride as a refractory hard material, and acarbonaceous aggregate, said aggregate containing anthracite and crushedanode butts or calcined coke with the crushed anode butts or calcinedcoke comprising up to 20% by weight of the total aggregate.
 2. A highswelling cold ramming paste according to claim 1 containing 10 to 15% byweight of said pitch.
 3. A high swelling cold ramming paste according toclaim 1 containing about 5 to 10% by weight of said light oil.
 4. A highswelling cold ramming paste according to claim 3 wherein the light oilhas a boiling point in the range of about 210-355° C.
 5. A high swellingcold ramming paste according to claim 1 wherein said pitch is present inan amount of 10 to 15% by weight, said light oil is present in an amountof 5 to 10% by weight.
 6. A method of making a high swelling coldramming paste for use in joining components of electrolytic cells forthe electrolysis of alumina for the production of aluminum whichcomprises mixing together pitch, light oil, 30 to 60% by weight oftitanium diboride as a refractory hard material, and a carbonaceousaggregate comprising anthracite and crushed anode butts or calcined cokecomprising up to 20% by weight of the total aggregate, said mixing beingcarried out at about 90 to 130° C. for about 20 to 50 minutes.
 7. Amethod according to claim 6, wherein said pitch is mixed in an amount of10 to 15% by weight, said light oil is mixed in an amount of 5 to 10% byweight.